A so-called two-component type heat-sensitive recording material using a color forming reaction between a colorless or slightly colored electron donating dye precursor and an electron accepting compound is disclosed, for example, in Japanese Patent Publication Nos. 14039/70 (corresponding to U.S. Pat. No. 3,539,375) and 4160/68. This type of two-component color forming heat-sensitive recording material is prepared by dispersing a colorless or slightly colored electron donating dye precursor and an electron accepting compount into a fine particle state, mixing a binder and the like therewith so that the electron donating dye precursor and the electron accepting compound are separated, and coating the mixture on a support. Upon heating, these heat-sensitive compounds melt and contact each other, resulting in a color forming reaction whereby recording takes place.
Such two component type color forming heat-sensitive recording materials are advantageous in that: (1) primary coloration takes place and therefore color development is unnecessary; (2) paper quality is similar to that of used for conventional types of recording; (3) handling is easy; (4) color density of the resulting images is high; and (5) upon color formation, various hues can be obtained. Accordingly, this type of recording material is very valuable. Therefore, this type of two-component color forming heat-sensitive recording material has become widely used recently, particularly in the fields of facsimile transmissions, recorders, and printers. With such increasingly wide usage in the field of facsimile transmissions, the recording rate has also increased significantly in recent years. Thus, in view of this tendency to increase facsimile recording rates, a strong demanded has arisen for heat-sensitive recording materials which have a short pulse, that is, the ability to undergo color formation with low energy input. In other words, improvement of heat reactivity of the recording materials has long been desired.
It has been suggested as one approach to satisfying this demand that the melting point of the electron accepting compound be raised from 60.degree. C. to 100.degree. C. However, it is difficult to adjust the melting point of phenol compounds, which are the most widely used of the known electron accepting compounds; further, such phenol compounds are costly and thus are not practical to use.
Another approach, disclosed in Japanese Patent Publication Nos. 17748/74 and 39567/76, is to use organic acids and phenol compounds in combination, or, alternatively, to use polyvalent metal salts of compounds having alcohol hydroxy groups as an electron accepting compound. Still another approach, disclosed in U.S. Pat. No. 3,859,112 and 3,936,309 is to employ a copolymer of hydroxyethyl cellulose and maleic anhydride as an electron accepting compound.
It is also disclosed in Japanese Patent Publication No. 27599/76 and Japanese Patent Application (OPI) No. 19231/73 (the term "OPI" as used herein means an "unexamined published application") that waxes may be added to the recording material to improve heat reactivity.
It is further known that a nitrogen-containing organic compound, such as thioacetoanilide, phthalonitrile, acetoamide, di-.beta.-naphthyl-p-phenylenediamine, fatty acid amide, acetoacetic anilide, diphenylamine, benzamide, or carbazole, a heat fusible substance such as 2,3-di-m-tolylbutane, 4,4'-dimethyl biphenyl, or a carboxylic acid ester such as dimethyl isophthalate, diphenyl phthalate, dimethyl terephthalate, may be used as a sensitizer, as described in U.S. Pat. Nos. 3,895,173 and 4,236,732, Japanese Patent Application (OPI) Nos. 115554/74, 149353/75, 106746/77, 5636/78, 11036/78, and 72996/81.
However, heat-sensitive recording materials which incorporate the above-described compounds are not completely satisfactory. Specifically, such heat-sensitive recording materials have defects in that density and heat responsiveness are not satisfactory. Also, fog formation occurs under high temperature and high humidity conditions, and fine powders appear on the surface of the heat-sensitive color forming layer with the passage of time, thus causing the color formed images to fade.